Water-cooled nuclear reactors, such as pressurized water nuclear reactors, comprise fuel assemblies which are placed in a juxtaposed disposition inside the reactor vessel, in order to form the core which is the center of nuclear reactions producing the energy which is transmitted to the coolant of the reactor in the form of heat.
The fuel assemblies are formed by bundles of lengthy fuel elements or rods of small diameter. The fuel rods themselves are formed by a metal cladding in which pellets of fuel material such as uranium oxide are stacked, and which is closed in a sealed manner at its ends by plugs.
The cladding of the fuel elements is constructed from an alloy resistant to high temperature corrosion by the water cooling the reactor and of low neutron absorption. The cladding of the fuel element makes it possible to isolate the pellets made of fuel material from the cooling water and to prevent fission products formed by nuclear reaction in the fuel material from being entrained by the water cooling the reactor.
After being present in the operating reactor for some period of time, certain assemblies of the core may exhibit sealing faults which are manifested by a release of radioactive fission products into the coolant. The level of radioactivity of the cooling water makes it possible to determine the presence of assemblies exhibiting leaks in the reactor core. These assemblies must be detected in order that their repair or their replacement can be effected, during a shutdown phase for maintenance of the reactor, so as to prevent contamination of the primary cooling system in which the cooling water flows.
The detection of the defective fuel assemblies may be effected under water in the pool, by acoustic, ultrasonic or eddy current methods of measurement.
It is also possible to use cells called sipping test containers in which the release of radioactive products by the fuel assembly is promoted, these radioactive products being subsequently carried away by the water or a gas into a unit enabling them to be detected.
In the case where such radioactive products have been detected, it is necessary to effect the replacement or the repair of the fuel assembly before reloading it into the reactor core. Leaks of radioactive product originating from an assembly arise from a sealing fault in at least one rod of the assembly whose cladding is cracked.
Certain methods for monitoring the fuel assemblies make it possible to locate the defective rod(s) but these methods are generally complex and do not make it possible to determine completely reliably the location of the defective rods.
Refurbishing defective assemblies, effected by replacing the rods which are likely to leak by new rods, is therefore not always effected under satisfactory conditions.
Furthermore, in the case of studies carried out in order to research the causes of the presence of faults in fuel elements, until now use has been made of eddy current test methods or macroscopic visual test methods carried out individually on each of the rods of the fuel assemblies subjected to the tests.
Until now no method and device making it possible to determine extremely reliably the presence of leaks from a fuel element of an assembly for a nuclear reactor, such as a fuel rod, has been known.